1. Field of the Invention
The present invention relates to electronically grounded heat spreaders employed in connection with the dissipation of heat, which is generated by electronic devices, and more particularly, relates to the unique adhesive fastening of metallic heat spreaders to semiconductor chips.
In particular, in the effectuation of adhesive attachments of metallic heat spreaders to the backsides of flip chips in order to dissipate heat from the latter, which is generated during operation of electronic devices employing the chips, it is an additional advantage to be able to electrically connect the flip chip to the heat spreader to gain in silicon performance if an electrical ground path can be established between the backside of the flip chip and the metallic heat spreader.
The aspects in compensating for any mismatches in coefficients of thermal expansion (CTE) and resultant contraction which are encountered between the various components of a wire bond or flip chip package or module including encapsulated semiconductor chips mounted on substrates, and with heat sinks supported on the chips in the form of heat spreaders, such as lids or caps, in order to reduce heat-induced warpage tending to separate the components and leading to failures of the electrical connects and ball grid arrays, adversely affecting the functioning and reliability of the packages has been widely addressed in the technology and industry. Nevertheless, notwithstanding the considerable efforts expended in order to solve the problems which are encountered, the utilization of the heat-spreading cap only balances the thermal dissipation above the substrate directly above the chip. The foregoing difficulties are encountered due to package warping as a result of thermal stresses generated, in that the normally utilized epoxy adhesives which cement the chip and cap may not match the coefficients of thermal expansion of the various components. One of the possible failure mechanisms is delamination of the epoxy interface between the chip and the cap as a result of thermally induced thermal stresses. Also, the tendency of the epoxy adhesive to absorb or desorb moisture may readily cause additional expansion and contraction and result in further warpage of the entire module structure, leading to an operative failure of the arrangement.
Currently, in the technology concerned with such electronic devices, adhesives which are constituted from electrically conductive epoxies (ECA) are a frequent choice in applications directed to the attaching of a heat spreader or a heat-dissipating lid or cap to the backside of a flip chip. However, the utilization of such electrically conductive epoxy adhesives is subject to various serious disadvantages. One such disadvantage is the inherently high modulus or coefficient of thermal expansion (CTE) of epoxies, which can readily range between several hundred thousand psi to over one million psi. Consequently, this class of epoxy materials is inherently subject to the weaknesses in that the adhesive bond between the chip backside and that of the heat spreader may be readily prone to fatigue due to the mismatch, which is encountered in the coefficients of thermal expansion (CTE) between the adhesive material and the metal lid or heat spreader, the latter of which is normally constituted of either copper or aluminum. Moreover, horizontal chip cracking can also be encountered at times due to an underfill laminate pulling away from the bottom surface of the chip at which location the chip is fastened to a substrate, such as a circuit board, laminate or ceramic material, and with the heat spreader pulling away at the top or the opposite surface of the chip.
As an alternative to employing the traditionally high modulus epoxies, which are utilized to adhesively interconnect the chips to the heat spreaders or metallic lids or caps fastened thereto, it is possible to employ low modulus epoxies, in effect, those of lower than one hundred thousand psi. From the standpoint of mechanical stress management, these epoxies fatigue less, and resultingly impart a lower stress causing a diminution of potential horizontal chip cracking. Nevertheless, they are subject to the drawback in that these epoxies are not heat resistant, whereas normal solder reflow temperatures required for BAT (Bond Assembly Testing) balling, of up to 250° C. and then card or substrate attachment will severely degrade adhesives formed of the low modulus epoxies, as well as any consisting of acrylics. Consequently, a loss of adhesion can be the result, as well as a degrading in thermal performance, thereby potentially rendering the entire adhesive interconnection between the heat spreaders or lids and the chips to be extremely unreliable, and the electronic device incorporating the latter as being essentially unusable.
In order to avoid the problems, which are presently being encountered in the technology, thermally conductive silicone adhesives have been employed in the attachment of heat spreaders to electronic components. However, although these adhesives comprise very low modulus materials and maintain their adhesive properties after demanding exposures to heat cycles, they are typically not electrically conductive. Thus, rendering silicone adhesives electrically conductive has not been successful; nevertheless recent advances in the development of electrically conductive silicones now render these adhesive materials a more viable choice for being employed in microelectronics.
2. Discussion of the Prior Art
In essence, utilizing the use of electrically non-conductive thermosetting silicone adhesives, which form a first type of connection in electronic devices of the type described, in conjunction with electrically conductive silicone adhesives forming electrical ground connections is disclosed in Kwon, et al., U.S. Pat. No. 6,518,660. In that instance, a substrate, preferably, but not necessarily constituted of a ceramic material, or which may also be a printed circuit board, is directly connected to a metallic heat spreader, which may consist of either aluminum or copper, through the intermediary of an electrically non-conductive thermosetting silicone adhesive. However, the surface of the chip facing the heat spreader is covered entirely with the electrically conductive epoxy or thermosetting electrically conductive silicone adhesive. Connected at and extending outwardly from the corners of the chip through suitable wiring is an electrically conductive silicone adhesive adapted to provide the appropriate electrical connection between the substrate and the chip and also the ground projections extending therebetween.
Furthermore, the utilization of an electrically conductive thermosetting silicone adhesive for microelectronics is also disclosed in a brochure of the Loctite Corporation, and which is intended to provide a resilient ground path for sensitive electronic devices, such commercially available electrically conductive bonding material or adhesive being identified as 5420 Heat Cure Silicone.
Although the Kwon, et al. U.S. Pat. No. 6,518,660 describes a semiconductor package with ground projections, wherein the front side of a flip chip attached in an integrated circuit to a substrate has an insulated underfill material, and wherein electrically conductive paths to ground points are provided with an electrically conductive epoxy adhesive, there is no provision of a heat spreader which is connected to a semiconductor chip by means of both electrically conductive and non-conductive silicone or epoxy adhesives in a manner pursuant to the invention.